A wavelength division multiplexed (WDM) cross-connect device, hereinafter referred to as a WDM cross-connect, is a network of fibers connected to various optical components that allow a set of input fibers to be connected to a set of output fibers. Each fiber in the network can support some fixed number n of wavelength channels. In other words, at any time there can be up to n signals along a fiber with each signal using a distinct wavelength. WDM cross-connects are capable of performing wavelength interchanging by connecting a wavelength channel on an input fiber to a different wavelength channel on an output fiber. WDM cross-connects comprise components that are capable of switching an incoming wavelength channel onto any different wavelength channel on an outgoing fiber. These components are commonly referred to as wavelength interchangers.
Another type of component comprised by WDM cross-connects is known as an optical switch. An optical switch has an arbitrary number of fibers passing into it and out of it and any wavelength channel on any incoming fiber can be switched to the same wavelength channel on any outgoing fiber, assuming the wavelength channel is not already being used. The WDM cross-connect also comprises optical fibers that are connected to the optical switches and to the wavelength interchangers at nodes. The optical fibers provide directed paths through the WDM cross-connect in the sense that the signal on any optical fiber only travels in a forward direction through the cross-connect and can never meet itself.
When a request for a connection in a WDM cross-connect is made, the WDM cross-connect must perform two fundamental tasks. First of all, a route or path must be found in the WDM cross-connect from the requested input fiber to the requested output fiber. Secondly, for each fiber in the route, an unused wavelength channel must be assigned so that (1) the wavelength channels assigned on the input and output fibers are the requested wavelength channels, and (2) the wavelength channels assigned on any two consecutive fibers in the route are the same, unless there is a wavelength interchanger connecting the two consecutive fibers.
WDM cross-connects have been proposed that have “non-blocking” properties. The term “non-blocking” corresponds to the ability of the WDM cross-connect to satisfy requests for connections, i.e., the requests are not “blocked” as a result of an unavailable route or wavelength channel. Some of these WDM cross-connects are rearrangeably non-blocking, which means that satisfying requests for new connections may require changing the paths and/or the wavelength channels of already-configured connections. In a WDM cross-connect, disrupting connections in order to create new connections is undesirable since doing so requires buffering of the connections that are to be rearranged.
A WDM cross-connect is considered to be pathwise rearrangeably non-blocking in cases where connection requests can be routed through the cross-connect, but any additional requests received after routing the original set of requests may require some of the previously routed requests to be re-routed. Some WDM cross-connects are considered to be pathwise wide-sense non-blocking. These WDM cross-connects employ a routing algorithm that enables any sequence of connection requests and withdrawals to be satisfied without disturbing any of the currently routed requests. Pathwise strictly non-blocking cross-connects are known that enable any set of requests to be routed through the cross-connect without disturbing the routes associated with previous requests.
A request for a connection requires not only a route from the input fiber to the output fiber, but also a wavelength channel assignment along the route that only changes wavelength channels at wavelength interchangers and that begins and ends on the requested wavelength channels. These requests for connections between wavelength channels on input and output fibers are commonly referred to as demands. When a demand is made following a previously routed demand, the routes and/or the wavelength channel assignments associated with the previously routed demands may need to be changed. The definitions of wavelength rearrangeably non-blocking, wavelength wide-sense non-blocking and wavelength strictly non-blocking are analogous to the definitions provided above for pathwise rearrangeably non-blocking, pathwise wide-sense non-blocking and pathwise strictly non-blocking, respectively.
A WDM cross-connect that is both pathwise and wavelength wide-sense non-blocking will be referred to hereinafter as a wide sense non-blocking WDM cross-connect. Although WDM cross-connects are known that are wide sense non-blocking WDM cross-connects, known designs require a relatively large number of wavelength interchangers. Since a substantial portion of the costs associated with WDM cross-connects is attributable to the costs of the wavelength interchangers, it would be desirable to provide a wide sense non-blocking WDM cross-connect that utilizes a minimum number of wavelength interchangers.
FIG. 1 is a block diagram of a WDM cross-connect 1 that is commonly referred to as a standard design WDM cross-connect. The fabric 2 between the input optical fibers 3 and the output optical fibers 4 of the WDM cross-connect 1 includes a plurality of nodes (not shown) and a plurality of optical fibers (not shown) that interconnect the nodes. Each of the nodes is comprised of a wavelength granularity switch that switches signals received by the fabric 2 on the input fibers 3 onto selected output fibers 4. The WDM cross-connect 1 comprises a controller 6 that controls the operations of the fabric 2 and of the wavelength interchangers 5. The controller 6 causes the wavelength granularity switches to select an appropriate output fiber 4 so that the wavelength of the signal routed onto the output fiber 4 will not be the same as the wavelength of a signal that already exists on the output fiber 4.
The WDM cross-connect 1 comprises k wavelength interchangers 5, where k is a positive integer equal to the number of input fibers 3 and output fibers 4. Each wavelength interchanger 5 is connected to a single input fiber 3. Each input fiber 3 is capable of simultaneously carrying signals at n wavelengths, λ1 through λn, where λ denotes wavelength and n is a positive integer. Therefore, each input fiber supports n wavelength channels. Each of the wavelength interchangers 5 is capable of permuting the wavelength of a signal on the input fiber 3 to a different wavelength. The fabric 2 then causes the signal to be routed onto a selected output fiber 4. The controller 6 controls the selection of the wavelength channels by the wavelength interchangers 5.
FIG. 2 is a block diagram of a WDM cross-connect 7 that is commonly referred to as a modified standard design WDM cross-connect. The WDM cross-connect 7 is a modification of the design shown in FIG. 1 and includes a wavelength interchanger 8 connected to each of the output fibers 4. The WDM cross-connect 7 comprises 2k wavelength interchangers. The additional wavelength interchangers 8 connected to the output optical fibers 4 enable the wavelength channel utilized by a signal on any of the output optical fibers 4 to be permuted. This enables demands to be handled that specify a particular output wavelength, which is not the case with the WDM cross-connect 1 shown in FIG. 1.
The additional wavelength interchangers 8 provide the WDM cross-connect 7 with improved versatility. However, 2k wavelength interchangers are utilized by the WDM cross-connect 7, which significantly increases the cost of the cross-connect in comparison to the cost associated with the cross-connect shown in FIG. 1. The cross-connects 1 and 7 are, at best, rearrangeably non-blocking and require a large number of wavelength interchangers.
Accordingly, a need exists for a wide-sense non-blocking WDM cross-connect design that minimizes the number of wavelength interchangers that are needed to provide the WDM cross-connect with wide-sense, non-blocking properties.